This invention relates to thermoelectric materials, and more specifically to bismuth nanocomposites exhibiting enhanced thermoelectric power.
Sub-ambient cooling is conventionally accomplished through gas/liquid vapor phase compression based refrigeration cycles using freon-type refrigerants to implement the heat transfers. Such refrigeration systems are used extensively for cooling human residences, foods, and vehicles. Sub-ambient cooling is also often used with major electronic systems such as mainframe server and workstation computers.
In 1974, it was first suggested that chlorofluorocarbon compounds, principally freon (R-12), the refrigerant of choice in air conditioning systems for the last 50 years, were destroying the protective ozone layer in the stratosphere at an alarming rate. The Montreal Protocol of 1987 has led to the gradual phasing out of these chemicals. Non-chlorine-containing fluorocarbons, such as R-134a, which do not possess the long-term stability of R-12, have since received widespread use as refrigerants. It was not realized until the late 1990""s, however, that all fluorocarbons, including both R-12 and R-134a, could contribute to global warming.
Thermoelectricity is one alternative climate control technology that does not contain such chemicals and presents many additional advantages, including all-solid-state operation, electronic capacity control, reversibility to provide both heating and cooling, and high reliability. Thermoelectric heat-to-electrical power converters also have potential uses in thermal energy recovery systems. However, thermoelectric cooling has not enjoyed widespread use because of its low efficiency relative to vapor compression systems. The thermoelectric efficiency (or coefficient of performance for a cooler) depends on the thermoelectric figure of merit, Z, of the material of which the thermoelectric device is comprised. This quantity is a combination of the thermoelectric power or Seebeck coefficient S, the electrical conductivity "sgr", and the thermal conductivity xcexa of the material:   Z  =                    S        2            ⁢      σ        κ  
Currently, the bulk material with the highest thermoelectric figure of merit at room temperature (300 K) is Bi2Te3 with Z≈0.003 Kxe2x88x921, but an improvement by about a factor of 2 is generally considered to be necessary for thermoelectric devices to be competitive with vapor-compression technology.
One approach to increasing Z is to search for physical systems exhibiting an enhanced thermoelectric power. In 1993, Hicks and Dresselhaus (xe2x80x9cThermoelectric Figure of Merit of a One-Dimensional Conductorxe2x80x9d, Physical Review B Vol. 47, pp. 16631, 1993) pointed out that, due to the quantum-mechanical nature of the motion of electrons through solids, confining such electrons in a structure with a physical dimension below the spatial extent of the electron wavefunction should result in an enhancement of Z. The main mechanism for the enhancement is due to an increase of the density of states near the Fermi level. As a result of the increase, a sufficient density of charge carriers can exist in the solid to maintain the electrical conductivity, but the Fermi energy is small, and this leads to a large S. An enhancement of Z has been observed in superlattices, but the inventors ascribe that effect to a control of the phonon thermal conductivity at the interfaces. Furthermore, superlattices are more suitable for thin-film applications than for large-scale climate control. Stronger increases in Z are predicted in quantum wires because of the additional confinement: the electronic density of states is characterized by the existence of peaks at quantized values of energy. Bismuth is a particularly attractive candidate, because the electronic effective mass is very small in this semimetal, and therefore the spatial extent of the electron wavefunctions is large. Furthermore, its lattice thermal conductivity is small, because it is the heaviest non-radioactive element. According to the theoretical basis for bismuth as a thermoelectric material, Bi nanowires become semiconductors when the diameter is decreased below 50 nm. An enhancement of the figure of merit to a value of ZT=6 (at 77K) is predicted for wires with 5 nm diameters, doped to 1018 electrons per cm3.
Multiple investigations using bismuth nanowires embedded in porous anodic alumina have followed the first theoretical calculation, mostly based on resistivity measurements. The first thermopower measurements on bismuth nanowires of 200 nm diameter embedded in anodic alumina did not show any enhancements in thermopower. The cause was identified to be the fact that 200 nm was too large a diameter for the nanowire effects to manifest themselves. The thermopower measurements for bulk Bi, 200 nm Bi/anodic alumina composites and an optimized Bi2Te3 alloy are shown in FIG. 1. The optimized Bi2Te3 alloy refers to an alloy of four bulk intermetallic compounds, Bi2Te3, Bi2Se3, Sb2Te3 and Sb2Se3 in accordance with the formula (Bi1xe2x88x92xSbx)2(Te1xe2x88x92ySey)3, with one or more dopants.
While the theoretical basis for the increase in S and Z has been developed, verification of these effects has been difficult to realize. The first hurdle is the sample synthesis technique. Molten bismuth has been inserted into porous hosts under very high pressures, but this technique is limited to wires with diameters greater than 40 to 50 nm, because the necessary pressure increases rapidly with decreasing diameter. Nevertheless, this technique has been used to prepare the first wires on which the semimetal/semiconductor transition was reported, as evidenced by magnetoresistance data. A new vapor-phase technique was developed and described in Thrush et al. U.S. Pat. No. 6,159,831 and, because it does not depend on the surface tension of the bismuth/host material interface, it has been used to prepare wires of diameters down to 7 nm. The temperature dependence of the electrical resistance of nanowires of diameters in the 200 nm to 7 nm range, and their magnetoresistance, illustrate the semimetal/semiconductor transition very clearly. Until now, the thermoelectric power was measured only on 200 nm wires embedded in anodic alumina, which are metallic and show no enhancement in S, as expected.
An additional obstacle to the realization of bismuth nanocomposites as a thermoelectric power source is the expense and impracticality of anodic alumina as the host material. Anodic alumina is provided as a thin film material, generally having a thickness less than 2 mil, and more typically on the order of 1 mil (0.0254 mm). These films are expensive, and their small size limits their ability to be used commercially for large-scale climate control.
There is thus a need for a bismuth nanocomposite exhibiting enhanced thermopower that may be practically produced for large-scale climate control applications.
The present invention provides a thermoelectric material that exhibits enhanced thermoelectric power, and thus an improvement in the thermoelectric figure of merit, thereby providing a commercially viable thermoelectric material for use as a thermoelectric power source. To this end, a bismuth-based material, as elemental bismuth, a bismuth alloy, a bismuth intermetallic compound, a mixture of these, or any of these including a dopant, is embedded in the pores of a porous host material having an average pore size in the range of about 5-15 nm. The host material is advantageously a non-anodic porous alumina, a porous glass or a porous silica-gel, which are available in bulk form. In an exemplary embodiment of the present invention, the host material is non-anodic porous alumina comprising porous xcex8xe2x80x94Al2O3 grains having an average pore size of about 9 nm fused together with non-porous xcex1xe2x80x94Al2O3 grains. In another exemplary embodiment, the host material is a porous silica-based material comprising at least about 90% silica having an amorphous or crystalline phase, for example silica-gel having an average pore size of about 15 nm or VYCOR(copyright) glass treated to have 5-15 nm pore sizes.
The present invention further provides a method of making a composite thermoelectric material, in which a host material, for example a non-anodic porous alumina, high purity silica glass or silica-gel, is provided having an average pore size of about 5-15 nm, and a vapor of a bismuth-based material is caused to flow into the pores. More specifically, the bismuth-based material is heated to form a vapor, and the vapor is flowed into the pores from a vapor inlet side of the host material to a vapor outlet side.
In an exemplary method, a heater is placed adjacent the vapor outlet side of the host material to heat the host material. The host material is then cooled from the vapor outlet side to progressively condense the vapor in the holes in the direction from the outlet side to the inlet side to progressively form nanowires of the bismuth material in the pores.